Semiconductor wafers use layers of semiconductor material, insulator material, and conductor material to build up integrated circuit patterns. These different layers can be formed by chemical vapor deposition, electroplating, or other means. For the specific use of bulk copper for next generation copper-based interconnects, the increasingly popular method of application is electroplating.
Referring to Prior Art FIG. 1A a top view of a prior art electrochemical cell used for electroplating a semiconductor wafer is presented. Similarly, Prior Art FIG. 1B is a side view of a prior art electrochemical cell presented in Prior Art FIG. 1A. The structure of the electrochemical cell will be explained herein. The electrochemical cell is typically constructed of a chamber 104 that encloses the balance of the electrochemical cell apparatus. In the cell is a semiconductor wafer 102 that acts as a cathode in the electrochemical operation. A copper anode 106 is disposed a distance away from semiconductor wafer 102. The semiconductor wafer 102 is coupled to leads 112. Similarly, copper anode 106 is coupled to leads 114. In between the anode 106 and semiconductor wafer 102 is a copper sulfate solution that fills chamber 104. The solution provides metal molecules in a liquid suspension. The subsequent electrical voltage and electrical current 108 applied across anode 106 and semiconductor wafer 102 cathode motivate the metal molecules to dissociate into metal ions which leave the solution to adhere to the semiconductor wafer 102 that acts as the cathode. The result is a deposited layer of film 116 composed of the metal that was previously in solution. More specifically, the film is a copper film 116.
Despite its popularity however, electroplating has several drawbacks. First, electroplating is a wet processing technique that is very sensitive to process variations. Consequently, the resulting copper film 116 has a thickness and surface that is uneven and inconsistent. Considering the tight tolerances involved in semiconductor wafer fabrication, a need exists to improve the crude and loosely controlled process of electroplating. More specifically, a need exists to control the variability of electroplating such that the plated metal film has an even and consistent thickness and surface.
One important variable in the plating process is the electrical current that drives the electroplating process. Because electrical current provides the driving force to propel metal ions in suspension towards the semiconductor wafer 102 cathode, controlling the variation in the electrical current will do much to control the thickness and uniformity of the electroplated metal film. Hence, a need arises for a method and apparatus that can reduce the variation in the electric field that drives the electroplating operation.
While the electric current may appear to be constant across the entire area spanned between the anode and the electrode, because a constant voltage is applied across both electrodes, in reality, the electrical current is not constant. Many factors, individually and together, alter and distort a theoretically constant electrical current that exists across the anode and cathode.
Some of the factors that alter and distort the electrical current include: variables changing over elapsed time of the electroplating operation; voltage variation across the semiconductor wafer 106 cathode; variation in the profile of anode 106 used in the electroplating operation; distortion caused by the chamber 104 housing the electrochemical operation; changes in the thickness of metal film 116 electroplated onto semiconductor wafer 102; and the electrical characteristics of the metal solution used in the electroplating operation. More specifically, temporal and voltage variations arise from sources such as changes to the metal solution conductivity, reduction of the resistivity of the semiconductor wafer cathode 106 as plated copper overtakes the copper seed layer, etc. Likewise, chamber 104 of electrochemical cell 100 may have an effect on the electrical current distribution. These and other examples illustrate the many sources of distortion on a theoretically constant electric current flux.
As an analytic example of the variation of the electrical current, a theoretical current used in a commercial electroplating cell would be calculated per: EQU I=(V*A)/(t*p);
where
A=area=.pi.r.sup.2 PA1 t=distance between anode and cathode PA1 .rho.=resistivity of metal solution used in the electroplating operation PA1 V=applied voltage across the cathode/ anode
By examining this equation, it is apparent that many factors can influence the resulting current calculation. For example, the distance between anode and cathode can vary due to erosion of the profile of the anode or due to thickness variations in the plated surface for the semiconductor wafer cathode. Many other similar such influences can be derived.
One way to improve the electroplating process, in view of these sensitivities, is to reduce the variations noted above. While this is possible, some variables are very difficult to control while others becomes exponentially difficult to control as their tolerances decrease. Consequently, a need arises for an apparatus and a method that will compensate for the variations in the electrical current and in other variables altering and distorting the electrical current for the electroplating operation.
In summary, a need exists for a method and system for improving the crude and loosely controlled process of electroplating. More specifically, a need exists to control the variable of electroplating such that the plated metal film has an even and consistent thickness and surface. Furthermore, a need arises for a method and an apparatus that can reduce the variation in the electric current distribution that drives the electroplating operation. Specifically, a need arises for an apparatus and a method that will compensate for the variations in the electrical current and in other variables altering and distorting the electrical current for the electroplating operation.